When equipping an oil or gas well or an injection well for production or other use, it is often required to use a downhole packer which is connected to the wellhead by tubing which is held under tension. The tension is required to hold the packer in the packed position, or to compensate for a change in length of the tubing if hot liquid or gases move up or down the tubing. Often, it is required to carry out the installation of this equipment while the well is under pressure from the geological zone of interest.
In a conventional tubing hanger system, once the downhole packer is in place, tension is then put on the tubing by pulling it. To install the tubing hanger on the well, the tubing string is stretched out of the tubing and held by slips. Once the tubing hanger has been installed, the slips are released and the tubing hanger is lowered into place in the conical section of the tubing spool and secured with dog screws. The packing ring is then tightened by turning in the packing bolts. The packing then seals the cavity in the hanger and the casing from anything above the tubing hanger.
When using a conventional tubing spool and tubing hanger system in a well under pressure, the packer and tubing are installed by an operation called "snubbing" where items are pushed into the well against the well pressure with suitable above ground equipment. In order to keep the pressure confined, blow out preventers are utilized. Once the tubing hanger has been installed on the tubing in the same manner as set out above, it is lowered through a first upper blow out preventer which has been opened to allow its passage into the annular space between the blow out preventers. During this operation, the second or lower blow out preventer is sealing the pressure in the well as the tubing slips through it. Once the blow out preventer is closed, the lower blow out preventer is opened to allow the passage of the dognut which is then seated in the conical part of the tubing spool. In order to set the packing, it is then necessary to remove the blow out preventer stack. There is always a problem at this stage as the pressure may not be sealed by the loose packing or primary packing. A second problem associated with the conventional tubing hanger system is that the stretch required to get the tubing through the blow out preventer stack is quite long and in shallow wells this becomes a critical factor in having the proper tension in the tubing with the tubing hanger in its final position in the hanger.